Two studies on the acoustics of multiphase materials : seagrass tissue and encapsulated bubbles

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Abstract

There are two focal points of this thesis: the acoustics of seagrass and the acoustical properties of encapsulated bubbles for underwater noise abatement. The acoustical properties of seagrass have applications in mine hunting, shallow water sonar, and environmental acoustic remote sensing. In order to optimize these applications, a predictive model of acoustic propagation in seagrass beds is sought. Previous laboratory research has indicated that the tissue acoustic properties of seagrass as well as the tissue physical structure and entrained air masses inside the leaves contribute to the overall acoustic behavior. The present research utilized a glass laboratory resonance tube to find the low frequency (1 kHz-4 kHz) acoustic compressibility of two species of seagrass, Thalassia testidinum and Halodule wrightii. By using a mixture of finely divided seagrass tissue suspended in seawater, the bulk moduli of the seagrass species were extracted. In the second section, encapsulated bubbles were analyzed as a method of abating underwater anthropogenic noise sources, since these sources, including marine piledriving and oil and gas exploration and production, pose potential harmful effects to marine life. Previous research, which used an array of rubber-shelled encapsulated bubbles, found the attenuation from these bubbles in be in close accordance with an existing encapsulated bubble model. Experiments were performed in a small laboratory resonance tank, a large outdoor acoustic tank, and at Lake Travis Test Station (LTTS) in order to determine the effects of varying an encapsulated bubble's wall thickness and fill material on bubble resonance frequency and damping. Results found that increasing the wall thickness tended to increase the balloon resonance frequencies measured in the small tank, which was strongly correlated to the frequency of maximum noise reduction in the large outdoor test tank and LTTS tests. The addition of polyester fibers and aluminum wool as fill materials decreased both the resonance frequency and quality factor, whereas helium-filled filled encapsulated bubbles had an increased resonance frequency but decreased quality factor as compared with air-filled bubbles. The resonance quality factor and void fraction further proved to affect the noise reduction near bubble resonance in the outdoor acoustic tank and LTTS tests. The measurements made with a single bubble in a small laboratory tank were correlated to measurements with a full-size system composed of many bubbles operating in open water.